Hot water heat is basically just as it sounds, hot water to heat your home. Hot water that is heated by a boiler unit, whether it be outside or inside of your home. The hot water is then used to heat your home by use of a heat exchanger, (which is placed in your existing ductwork) or the water could be run through existing radiators, or in floor piping. The water completes its cycle by returning to the unit to get reheated, unless you wish to heat your garage as well. Shop style heat exchangers are available of that application. The water would run through your heat exchange in your garage and then return to the unit. If an adequate sized heater is used, you can heat your home, garage and possibly another building. The water moves in a constant cycle staying at the temperature your thermostat demands ensuring heat when you need it. Whether inside or outside, the unit must be connected by use of insulated piping. There are a variety of grades of insulated piping that ensure you can find the quality you desire. It is important to get the best piping you can because the better the grade the less heat loss in between the unit and heat exchange. Outside units are even more dependent on better piping because the piping is run underground at least 15' which is the minimum distance an outside unit can be from any dwelling. Hot water heat can also be used to heat your hot water through use of an optional side arm or plate exchanger. Boiler Units can be connected to a thermostat so they will act accordingly. If the unit for some reason stops working, your existing furnace would be the back-up and pick up where the boiler left off. This ensures you are never without heat, which takes the worry out of leaving home for an extended period of time. 
